The present invention relates to digital communications receivers, and, more particularly, to a novel digital communications receiver for recovering data phase-shift modulated on to a communications system carrier waveform.
It is known that digital information may be transmitted in a communications system by phase-shift modulation, wherein the phase of a carrier waveform is inverted, i,e, shifted by 180.degree., during each bit time interval in which a first binary data value is to be transmitted, and is maintained with the same phase as the immediately preceding bit time interval, when the remaining binary data value is to be sent in a particular bit time interval. It is also known that transmission of digital information may occur using a powerline, between a central facility and a multiplicity of remote locations each tied to the power distribution network, as the transmission medium. Powerline communication systems of this type are disclosed and claimed in U.S. Pat. Nos. 3,973,087 and 3,973,240, both issued Aug. 3, 1976; U.S. Pat. No. 3,944,723, issued Mar. 16, 1976; U.S. Pat. No. 4,135,181, issued Jan. 16, 1979; and U.S. Pat. No. 4,161,720, issued July 17, 1979, all assigned to the assignee of the present application and all incorporated herein in their entirety for reference. In powerline communication systems, wherein a powerline frequency (typically 60 Hertz (Hz.) in the United States) signal is present throughout the entire system, it is also known that the signal-to-noise ratio of the received signal can be greatly improved if the transmitted carrier signal frequencies are odd multiples of the first sub-harmonic of that powerline frequency. Methods and apparatus for realizing such improvements in signal-to-noise are described in claim U.S. Pat. No. 4,101,834, issued July 18, 1978; and U.S. Pat. No. 4,109,204, issued Aug. 22, 1978, both assigned to the assignee of the present application and incorporated herein by reference in their entirety.
In a data communications system in which the carrier is continuously present, even when the carrier is not modulated to transmit digital data to a receiving location, one presently known receiver filtering out both systematic pulse and random noise, to improve the signal-to-noise ratio, is described and claimed in U.S. Pat. No. 3,944,932, issued Mar. 16, 1976 to the assignee of the present invention and incorporated herein by reference in its entirety. This receiver utilizes at least one narrow band-pass commutating filter preceeded by at least one carrier-amplitude-limiting stage. The commutating filter requires a large number of precision resistance and capacitance elements, having relatively high cost, and also requires careful adjustment of a number of control elements during assembly, further adding to production cost. A receiver having high rejection of undesirable signals, but also having a relatively low production cost, is highly desirable.